A Word to the WISE: Confusion is Unavoidable for WISE-selected Infrared Excesses (2002.01021v1)

Abstract: Stars with excess infrared radiation from circumstellar dust are invaluable for studies of exoplanetary systems, informing our understanding on processes of planet formation and destruction alike. All-sky photometric surveys have made the identification of dusty infrared excess candidates trivial, however, samples that rely on data from WISE are plagued with source confusion, leading to high false positive rates. Techniques to limit its contribution to WISE-selected samples have been developed, and their effectiveness is even more important as we near the end-of-life of Spitzer, the only facility capable of confirming the excess. Here, we present a Spitzer follow-up of a sample of 22 WISE-selected infrared excess candidates near the faint-end of the WISE detection limits. Eight of the 22 excesses are deemed the result of source confusion, with the remaining candidates all confirmed by the Spitzer data. We consider the efficacy of ground-based near-infrared imaging and astrometric filtering of samples to limit confusion among the sample. We find that both techniques are worthwhile for vetting candidates, but fail to identify all of the confused excesses, indicating that they cannot be used to confirm WISE-selected infrared excess candidates, but only to rule them out. This result confirms the expectation that WISE-selected infrared excess samples will always suffer from appreciable levels of contamination, and that care should be taken in their interpretation regardless of the filters applied.

• The paper demonstrates that Spitzer follow-up of 22 WISE-selected candidates confirmed only 14 true infrared excesses, highlighting a high rate of source confusion.
• The study evaluates methods like ground-based near-infrared imaging and astrometric filtering, revealing limitations in resolving contamination due to WISE's coarse resolution.
• The research underscores the need for advanced deblending algorithms and cross-mission data integration as Spitzer's capabilities wane, to improve exoplanetary and circumstellar disk catalogs.

Insights into WISE-Selected Infrared Excesses and Their Confusion with Source Contamination

The paper "A Word to the WISE: Confusion is Unavoidable for WISE-selected Infrared Excesses" addresses a significant challenge in identifying infrared excesses due to circumstellar dust around stars when using data from the Wide-field Infrared Survey Explorer (WISE) mission. The authors, Dennihy et al., conducted a paper involving the follow-up of WISE-selected infrared excess candidates using the Spitzer Space Telescope to evaluate the prevalence and impact of source confusion on sample purity.

Key Observations and Findings

The paper examines a sample of 22 WISE-selected infrared excess candidates near the faint detection limits of WISE. Notably, only 14 candidates were confirmed to have true infrared excesses as verified by Spitzer's higher-resolution imaging, while 8 candidates showed excesses due to source confusion. This result underscores the substantial contamination rate and highlights the limitations of WISE owing to its coarse spatial resolution, which is prone to mixing signals from adjacent sources.

The authors explore the efficiency of various techniques to limit contamination:

1. Ground-based Near-infrared Imaging: This method proves inadequate for confirming candidates as truly possessing infrared excess due to undetected sources that contribute to contamination. Nevertheless, it remains valuable for eliminating obvious cases of source confusion before expensive space-based follow-up.
2. Astrometric Filtering: By leveraging proper motion data and the positional offsets between WISE and Gaia's higher-precision astrometry, the paper uses the Gaia-ALLWISE cross-match to develop a "Figure of Merit" (FoM). The analysis indicates that candidates exhibiting high WISE signal-to-noise ratios but low FoM scores are likely confused. However, several true excesses would be erroneously rejected if this criterion alone was applied, emphasizing the need for caution with such filtering.
3. Proper Motion Comparisons: Although the NEOWISE mission provides extended temporal baselines enabling refined proper motion measurements, this check is only viable for bright objects or those with appreciable motion. Limited applicability restricts its utility in large-scale surveys.

Theoretical and Practical Implications

The paper warns against reliance on WISE data for cleanly resolving samples of infrared excesses without caution or confirming observations. This caution is particularly essential as WISE, being confusion-limited, does not offer spatial resolution capable of disentangling close-proximity astrophysical phenomena, leading to a potential inflation in false positives. Researchers are advised to interpret statistical analyses of WISE-derived samples with a bias adjustment for assumed contamination levels.

The future of astronomical survey methodologies must revolve around the integration of advanced deblending algorithms and the use of synergistic datasets (like Gaia's astrometry) for validation. The end-of-life of the Spitzer mission, a major verification tool, further exacerbates the need for such strategies until future missions like the James Webb Space Telescope (JWST) can supplement this critical capability, albeit with its operational constraints.

Conclusions and Future Outlook

This research contributions forge a clearer path toward addressing the prevalent issue of sample contamination in WISE-selected circumstellar studies. As the astronomical community seeks greater fidelity in such detections, combining cross-mission data and enhancing algorithmic filtering will be paramount. Ongoing advancement in these domains likely heralds more accurate catalogs of exoplanetary and circumstellar disk phenomena, essential components for understanding planetary system formation and evolution across the cosmos.